A conventional head for a magnetooptic disk has been described on pages 3972 to 3978 of "APPLIED OPTICS, Vol. 23, No. 22, 15 Nov., 1984". The conventional head comprises a semiconductor laser for radiating laser beam, a collimating lens for collimating the laser beam, an objective lens for focusing the laser beam on a disk, two beam splitters positioned between the collimating and objective lenses, and two means corresponding to the two beam splitters for detecting the polarization rotation of the laser beam reflected from the disk and for detecting a position error of a beam spot on the disk.
In operation, the laser beam which is radiated from the semiconductor laser is collimated by the collimating lens, then passed through the two beam splitters, and focussed on the disk by the objective lens. The laser beam is a linearly polarized light which is focussed to a minute beam spot on the disk. The light beam reflected from the disk is introduced to the two detecting means by the two beam splitters, respectively. In the means for detecting polarization rotation of the reflected laser beam, a direction of magnetization in the disk is detected in accordance with the difference of light amounts between P and S polarized lights which are divided therein as described in more detail later. In the means for detecting a position error of a beam spot on the disk, on the other hand, focus and track errors are detected in accordance with the knife edge and push-pull methods as also described in more detail later.
Another conventional head for an optical recording medium has been described in Japanese patent application No. 61-14144. The conventional head comprises a semiconductor laser for radiating a laser beam, an objective lens for focusing the laser beam on a disk, a grating means for diffracting the laser beam which is reflected from the disk, the two, two-divided detecting means for detecting focus and track errors. The grating means includes upper and lower planes, one having a different pitch of gratings from other.
In operation, the laser beam radiated from the semiconductor laser is focussed to a minute beam spot on the disk by the objective lens. The laser beam which is reflected from the disk is partially diffracted by the upper and lower planes of the grating means. The light beams thus diffracted in different directions are received in the two, two-divided detecting means so that focus and track errors are detected in accordance with the knife edge and push-pull methods, respectively.
In general, a magnetooptic recording system provides high density recording found in optical recording systems and a re-writing ability found in magnetic recording systems so that it can be applied to a system in which a file having a large capacity is included. As described before, information is read from the disk in accordance with a magnetic Kerr effect. That is, a polarized plane of light reflected from the disk is rotated dependent on a direction of magnetization in the disk when a linearly polarized light is supplied to the disk.
According to the conventional head first discussed herein however, the means for detecting polarization rotation comprises a half wave plate, a polarized beam splitter, a pair of focusing lenses and so on, and the means for detecting a position error of a beam spot on the disk comprises a focusing lens, a beam splitter and so on, so that the construction of the head is complicated and the size thereof is not small.
According to the conventional head second discussed herein, further, there is the disadvantage that means for detecting polarized light rotation is complicated and is not small for the same reason as in the first discussed conventional head, although means for detecting a position error of a beam spot on the disk is simplified by using a grating means as described above.